Lemma 2.1 ([1]} Corollary 3.2, [2]} Theorem 8.5) 
Assume that \(A\)  satisfies (REF ) and (REF ) in \(\mathbb {R}_+^{n+1}\) , and define \(L\)  as in (REF ). Assume that there is some uniform constant \(C<\infty \)  such that for all Borel sets \(H\subset \mathbb {R}^n\) , the weak solution \(u\)  to the Dirichlet problem
\({\left\lbrace \begin{array}{ll}Lu=0 \quad \text{in }\mathbb {R}_+^{n+1}\\u=\chi _H \quad \text{on }\partial \mathbb {R}_+^{n+1}\end{array}\right.}\) 
